import org.eclipse.jface.window.ApplicationWindow;
import org.eclipse.swt.SWT;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Control;
import org.eclipse.swt.widgets.Display;

public class MainWindow extends ApplicationWindow {
    public MainWindow() {
        super(null);
    }

    @Override
    protected Control createContents(Composite parent) {
        Button btn = new Button(parent, SWT.NONE);
        btn.setText("OK");
        return parent;
    }

    public static void main(String[] args) {
        MainWindow w = new MainWindow();
        w.setBlockOnOpen(true);  // Window を閉じるまで open メソッドをブロック
        w.open();
        Display.getCurrent().dispose();
    }
}